1. Field of the Invention
The present invention relates to image forming apparatuses such as a printer apparatus, a copying machine, a facsimile machine and a complex machine having functions of those. More specifically, it relates to an image forming apparatus having a spectrum spreading clock generator (SSCG) which spreads a clock signal having a predetermined standard frequency at a predetermined spreading ratio and outputs the same.
2. Description of the Related Art
There has been an image forming apparatus, such as a printer apparatus, a copying machine, a facsimile apparatus, and a complex machine having functions of those, having an SSCG (abbreviation of: Spread Spectrum Clock Generator) which spreads a clock signal having a predetermined standard frequency at a predetermined spreading ratio and outputs the same (for example, refer to Japanese Unexamined Patent Publication No. 2004-358740).
With use of the SSCG, a frequency of a clock signal used as an operation clock of various control circuits provided in the image forming apparatus is slightly changed, thereby lowering a peak value of a frequency spectrum and reducing a radiation noise.
Clock modulating methods of the SSCG include a center spread, which allows a clock signal having a standard frequency of 20 MHz to spread within a range of ±0.5% i.e. between 19.9-20.1 MHz, and a down spread, which allows a clock signal to spread within a range of −1% down to 19.8 MHz where the standard frequency of 20.0 MHz is maximum. At this time, a theoretical average value of the frequency of the clock signal in the center spread is 20.0 MHz, and a theoretical average value of the frequency of the clock signal in the down spread is 19.9 MHz.
However, depending on a precision error, characteristics, and use environment for each part of the SSCG, distortion may occur in a frequency of a clock signal outputted from the SSCG. Therefore, in a case where a clock signal outputted from the SSCG is used as an operation clock signal for a control circuit, an effect to a time-counting function of an internal counter of the control circuit is concerned.
In particular, in a tandem-type image forming apparatus which forms a color image by sequentially superimposing toner images of respective colors, which are formed on a plurality of photosensitive drums, onto a transfer body such as an intermediate transferring belt, a sheet, or the like, a resist correction processing for correcting misalignment in transfer positions of toner images is executed. In this resist correction processing, a counter value of a clock signal is used when detection intervals of patch images of respective colors transferred from photosensitive drums for respective colors onto the transfer body are measured. Therefore, in a case where distortion occurs in the frequency of the clock signal, detection intervals of patch images cannot be measured accurately, thereby causing a problem that correction precision in the resist correction processing is lowered.